A team of US researchers has revealed the global spread of an ancient group of retroviruses that affected about 28 of 50 modern mammals' ancestors some 15 to 30 million years ago.
Retroviruses are abundant in nature and include human immunodeficiency viruses (HIV-1 and -2) and human T-cell leukemia viruses.
The findings on a specific group of these viruses called ERV-Fc show that they affected a wide range of hosts, including species as diverse as carnivores, rodents and primates.
The distribution of ERV-Fc among these ancient mammals suggests the viruses spread to every continent except Antarctica and Australia, and that they jumped from one species to another more than 20 times, said the team from Boston College, US.
The study also places the origins of ERV-Fc at least as far back as the beginning of the Oligocene epoch, a period of dramatic global change marked partly by climatic cooling that led to the Ice Ages.
Vast expanses of grasslands emerged around this time, along with large mammals as the world's predominate fauna.
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"Viruses have been with us for billions of years, and exist everywhere that life is found. They have a significant impact on the ecology and evolution of all organisms, from bacteria to humans," said co-author Welkin Johnson, professor of biology, in a paper forthcoming in the journal eLife.
Unfortunately, viruses do not leave fossils behind, meaning we know very little about how they originate and evolve.
"Over the course of millions of years, however, viral genetic sequences accumulate in the DNA genomes of living organisms, including humans, and can serve as molecular 'fossils' for exploring the natural history of viruses and their hosts," Johnson added.
The studies also allowed the team to pinpoint patterns of evolutionary change in the genes of these viruses, reflecting their adaptation to different kinds of mammalian hosts.
Perhaps most interestingly, the researchers found that these viruses often exchanged genes with each other and with other viruses, suggesting that genetic recombination played a significant role in their evolutionary success.
"Mammalian genomes contain hundreds of thousands of ancient viral fossils similar to ERV-Fc," noted lead author William E Diehl from University of Massachusetts.